Thermal barrier for turbine blades, having a columnar structure with spaced-apart columns

ABSTRACT

A process for depositing a ceramic layer on a metal substrate for producing a thermal barrier, the process including depositing the ceramic in a columnar structure. The deposition is carried out through a grid pierced with holes, which is positioned parallel to a surface of the substrate so as to produce ceramic columns separated from one another by a space. The process can further include a subsequent depositing of an isotropic ceramic layer in the spaces.

The field of the present invention is that of turbomachines and, more particularly that of components for these turbomachines which are subjected to high temperatures.

A turbomachine, as used for propulsion in the aeronautical field, comprises an atmospheric air intake that communicates with one or more compressors, generally including a fan, which are rotated about one and the same axis. The main stream of this air, after having been compressed, supplies a combustion chamber positioned annularly around this axis and is mixed with a fuel in order to provide hot gases, downstream, to one or more turbines through which these hot gases are expanded, the turbine rotors driving the rotors of the compressors. The engines operate at a temperature of the engine gases at the turbine inlet which is sought to be as high as possible because this temperature conditions the performances of the turbomachine. For this purpose, the materials of the hot sections are selected to withstand these operating conditions and the walls of the components swept by the hot gases, such as the turbine nozzles or the rotating turbine blades, are provided with cooling means. Furthermore, due to the metallic structure of these blades, made of a superalloy based on nickel or on cobalt, it is also necessary to protect them against the erosion and corrosion which are generated by the constituents of the engine gases at these temperatures.

Among the protections devised for enabling these components to withstand these extreme conditions is the deposition of a coating, referred to as a thermal barrier, on their outer face. A thermal barrier is generally composed of a ceramic layer of around a hundred microns, which is deposited at the surface of the metallic layer. An aluminum sublayer, of a few tens of microns, placed between the ceramic and the metallic substrate, completes the thermal barrier by providing the connection between these two components and also the protection of the underlying metal against oxidation. This aluminum sublayer, which is generally deposited by a vapor phase aluminization process (referred to as APVS for the version of the process used by the applicant), is fastened to the substrate by metallic interdiffusion and forms a protective oxide layer at the surface. An example of the implementation of this technique is described in patent application FR 2928664 by the applicant.

As regards the actual thermal barrier, made of ceramic, it may be produced in several ways, depending on the use which will be made thereof. Two types of structures are roughly distinguished for thermal barriers: columnar barriers, the structure of which is that of columns juxtaposed next to one another and which extend perpendicular to the surface of the substrate, and laminar or isotropic barriers that extend in uniform layers over the surface of the substrate.

The first ones are generally produced by a process referred to as EBPVD (electron beam physical vapor deposition) in which a target anode is bombarded, under high vacuum, by an electron beam emitted by a charged tungsten filament. The electron beam makes the molecules from the target pass into the gaseous phase. These molecules then precipitate in a solid form, covering the part to be protected with a thin layer of the anode material. These thermal barriers are characterized by a good resistance to thermal cycling but also by a relatively high thermal conductivity.

The isotropic barriers are generally deposited by plasma, using a thermal spraying process of the APS (atmospheric plasma spraying) type or by a sol-gel process. The sol-gel process makes it possible, via a simple polymerization of molecular precursors in solution, to obtain, at a temperature close to ambient temperature, glassy materials without passing through a melting step. These precursors exist for a large number of metals and are, for the most part, soluble in standard solvents. In this liquid phase that is denoted under the name of sol, the chemical reactions contribute to the formation of a three-dimensional inorganic network, known under the name of gel, in which the solvent remains. The process of obtaining the material, from the gel, passes through a drying step which consists in evacuating the solvent out of the polymer network. The advantage of such a barrier is the porosity that it exhibits.

The isotropic barriers are therefore characterized by a low thermal conductivity, which is the desired objective, but they have an inadequate resistance to thermal cycling. The barriers obtained by the sol-gel process have, themselves, a mediocre erosion resistance.

Finally, multi-fissured thermal barriers are known, which are obtained by plasma using a process described in several patents by the applicant (EP 1 645 654 and EP 1 471 162), which exhibit an acceptable compromise between the service life and the erosion resistance.

All these barriers are not however sufficiently high-performance and it is necessary to further improve their performances in these two domains.

The objective of the present invention is to overcome these drawbacks by proposing a process for producing a thermal barrier which does not comprise some of the drawbacks of the prior art and, in particular, which has a low conductivity combined with a good service life.

For this purpose, one subject of the invention is a process for depositing a ceramic layer onto a metallic substrate for producing a thermal barrier, comprising a step of depositing said ceramic in a columnar structure, characterized in that said deposition is carried out through a grid pierced with holes, positioned parallel to the surface of the substrate so as to produce at least two columns of ceramic which are separated from one another by a spacing.

The columns thus produced are sufficient to ensure the mechanical strength of the barrier and its erosion resistance and leave, furthermore, space between them in order to fill the latter with the most appropriate material. The invention thus creates a great flexibility for the composition of the thermal barrier.

Advantageously, the width of the holes is between 10 and 300 microns.

Preferably, the spacing between the holes is between 10 and 100 microns.

In one particular embodiment, the process also comprises a subsequent step of depositing an isotropic layer of ceramic in said spacings.

The isotropic structure of the deposit in the spacings guarantees a good impermeability of the barrier against the invasion of oxidizing gases from the stream in the direction of the substrate.

Advantageously, the second deposition is carried out by an operation for dip-coating the substrate equipped with its columns into a solution of sol-gel type.

A ceramic with an isotropic structure is thus obtained, which has a high porosity and therefore a low thermal conductivity.

Preferably, the isotropic deposition is carried out by a sequence of dip-coating and withdrawal operations in said sol-gel solution and drying operations carried out between two dip-coating and withdrawal operations, until a thickness substantially equal to the height of the columns is obtained.

In this configuration, the columns ensure both a good mechanical strength and a protection of the isotropic layer.

Advantageously, the process also comprises a final step of heat treatment.

The invention also relates to a thermal barrier deposited on a metallic substrate, characterized in that it comprises ceramic columns that extend perpendicular to the surface of said substrate and that are separated from one another by spacings, said spacings being filled with an isotropic ceramic layer.

Advantageously, the columns have a maximum width of between 10 and 300 microns.

Preferably, the spacings have a width of between 10 and 100 microns.

In one particular embodiment, the isotropic layer is made of porous ceramic.

The invention finally relates to a turbine blade for a turbomachine comprising a thermal barrier as described above and to a turbomachine comprising at least one such blade.

The invention will be better understood, and other objectives, details, features and advantages thereof will become more clearly apparent in the course of the detailed explanatory description which follows of an embodiment of the invention given by way of purely illustrative and non-limiting example, with reference to the appended schematic drawings.

In these drawings:

FIG. 1 is a schematic view of the physical composition of a thermal barrier for a turbine blade;

FIG. 2 is a schematic cross-sectional view of a thermal barrier after carrying out a first step of a process according to one embodiment of the invention;

FIG. 3 represents the four phases for carrying out the second step of the process according to one embodiment of the invention;

FIG. 4 is a schematic cross-sectional view of a thermal barrier at the end of the process according to the invention.

With reference to FIG. 1, seen in cross section is the composition of a thermal barrier deposited on the surface of a turbine blade, the latter being based by a stream of hot gas represented by an arrow pointed toward the left of the figure. The metal constituting the blade, typically a superalloy based on nickel or cobalt, forms a substrate 1, deposited on which is a sublayer made of aluminum 2, sandwiched between the substrate 1 and a ceramic layer 3. The role of the aluminum sublayer is to retain the ceramic layer and to offer a certain elasticity to the assembly in order to enable it to absorb the difference in expansion, represented by two arrows in opposite directions, that exists between the high-expansion substrate 1 and the low-expansion ceramic 3.

The ceramic 3 represented here is of columnar structure, which allows lateral displacements, owing to the appearance of cracks between the columns, and which gives it a good service life. The aluminum is then brought into contact with the oxygen conveyed by the gases that circulate in the stream of the turbomachine, which results in an average thermal conductivity of the barrier and a gradual damaging thereof.

Referring now to FIG. 2, the progress of the production of a thermal barrier after the implementation of the first step of the process according to the invention is seen. Placed on top of the substrate 1 to be covered is a grid 10 formed of evenly-spaced holes 11 so as to let through the vapor-phase deposition carried out by the EBPVD process or by any other process enabling the production of a columnar deposition (such as for example the APS process under very low pressure, carried out by the company Sulzer and known under the name LPPS-TF). The grid forms a mask which enables the deposition of the ceramic in the form of columns or of a group of columns 5 spaced apart from one another. The spacing thereof is, on the one hand, large enough so that a subsequent inter-columnar deposition can be carried out and, on the other hand, close enough to guarantee the mechanical strength of the whole of the thermal barrier. Typically, the columns or the groups of columns 5 have a thickness between 10 and 300 microns and the spacing 6 between them varies between one and a few tens of microns.

At the end of this first step, the thermal barrier is in the situation represented, with a substrate 1 and a sublayer 2 which are surmounted by an assembly of columns 5 made of ceramic. These columns conventionally have a shape which gets wider towards the top and which results from the gradual aggregation of the particles deposited. Between these columns are empty spaces which will be filled during the second step of the process according to the invention.

FIG. 3 shows, in four diagrams referenced 3 a to 3 d, the carrying out of this second step. Each diagram corresponds to a phase during which:

1-phase 3 a: the substrate equipped with its ceramic columns 5 is dip-coated in a solution 20 of sol-gel type based in particular on precursors of yttriated zirconia, which is used in the processes for producing an isotropic thermal barrier. The viscosity of the solution is such that it is sufficiently fluid in order to be able to be inserted into the spacing 6 between the columns 5 and fill them completely, and it is sufficiently viscous so that it remains stuck to the component during the withdrawal thereof;

2-phase 3 b: the component to be covered remains submerged in the solution 20 long enough for the spacing 6 between the columns 5 to be correctly filled;

3-phase 3 c: the component is then withdrawn from the solution 20 at a controlled speed so that a film of a desired thickness can be formed at the surface of the thermal barrier, homogeneously and with good adhesion;

4-phase 3 d: it is dried so that the solution 20 which has remained trapped between the columns 5 solidifies. After drying and removal of the solvent, a thin layer of ceramic is obtained which remains lodged between the columns. Since the thickness of ceramic deposited during the fourth phase is very small, it is necessary to carry out the operation, known as dip-coating, several times, that is to say to repeat the four operations after the drying of each of the layers formed in 3 d.

FIG. 4 gives the result obtained after repetition of the four operations from FIG. 3. The substrate 1 and its sublayer 2 are covered with a thermal barrier 3 composed of evenly-spaced columns 5, between which ceramic is deposited in isotropic form 7. This isotropic layer has many air bubbles that are trapped, which gives it a high porosity, and also gives the thermal barrier a good resistance to heat conduction.

The procedure of the process for producing a thermal barrier according to the invention will now be described.

The substrate constituting the material of the blade to be protected is first covered with a sublayer made of aluminum or of any other metal capable of constituting a thermal barrier sublayer. It is placed in equipment for the deposition of a ceramic layer, for example by electron beam physical vapor deposition, by positioning a grid 10, pierced with holes 11, on top of the component to be protected, at a distance that enables the formation of ceramic columns or a group of ceramic columns. The deposition takes place through holes 11 and the ceramic is deposited on the substrate 1 by growing perpendicularly to said substrate. Due to the mask generated by the solid parts of the grid 10, the deposition takes place along columns 5 distributed discretely over the surface of the substrate 1; between these columns 5 empty spacings 6 remain, which will be filled during the next step of the process. The component to be protected is then withdrawn from the columnar deposition equipment and transferred to a second piece of equipment for the deposition of the porous portion.

The second step of the process consists of a succession of dip-coating operations in a sol-gel type solution, comprising the four phases described previously. During each of these operations, the spacings 6 are filled with a thin layer of porous ceramic which accumulates, dip coating after dip coating, until a layer 7 is formed that completely fills the spacings 6.

The production of the thermal barrier is completed by a conventional heat treatment, during which the ceramic is stabilized and acquires the desired crystalline structure.

Finally, a mixed thermal barrier is obtained that comprises, on the one hand, a series of columns 5 which ensure a good mechanical strength and a good resistance to erosion by the gases which sweep over the component, and, on the other hand, a highly porous isotropic layer which ensures a good resistance to thermal conduction in the direction of the substrate. This protects the substrate 1 and the sublayer 2 against oxidation by the gases from the stream that circulates in the engine. Furthermore, the presence of columns enables the thermal barrier to spread out longitudinally over the surface of the substrate, during the expansion thereof, without risking the appearance of cracks which would enable oxygen from the gases to reach the metal of the substrate and damage it.

The objective of having a thermal barrier which combines a low thermal conductivity, a good erosion resistance and a good adaptation to thermo mechanical stresses, is thus achieved.

The first step of the production of the thermal barrier was described using the EBPVD process, but it can just as well be carried out with the other known deposition processes, such as thermal spraying, the presence of the mask formed by the grid being sufficient to generate the desired columnar structure during this step. 

1-12. (canceled)
 13. A process for depositing a ceramic layer onto a metallic substrate for producing a thermal barrier, comprising: depositing the ceramic in a columnar structure, the depositing taking place through a grid pierced with holes, positioned parallel to a surface of the substrate so as to produce at least two columns of ceramic that are separated from one another by a spacing; and a subsequent depositing an isotropic layer of ceramic in the spacings.
 14. The process as claimed in claim 13, wherein a width of the holes is between 10 and 300 microns.
 15. The process as claimed in claim 13, wherein the spacing between the holes is between 10 and 100 microns.
 16. The process as claimed in claim 13, wherein the second depositing is carried out by an operation for dip-coating the substrate including its columns into a solution of sol-gel type.
 17. The process as claimed in claim 16, wherein the isotropic depositing is carried out by a sequence of dip-coating and withdrawal operations in the sol-gel solution and drying operations carried out between two dip-coating and withdrawal operations, until a thickness substantially equal to a height of the columns is obtained.
 18. The process as claimed in claim 13, further comprising a heat treatment.
 19. A thermal barrier deposited on a metallic substrate, comprising: ceramic columns that extend perpendicular to a surface of the substrate and that are separated from one another by spacings, the spacings being filled with an isotropic ceramic layer.
 20. The thermal barrier as claimed in claim 19, wherein the columns have a maximum width of between 10 and 300 microns.
 21. The thermal barrier as claimed in claim 19, wherein the spacings have a width of between 10 and 100 microns.
 22. The thermal barrier as claimed in claim 19, wherein the isotropic layer is made of porous ceramic.
 23. A turbine blade for a turbomachine comprising a thermal barrier as claimed in claim
 19. 24. A turbomachine comprising at least one turbine blade as claimed in claim
 23. 